The present invention relates to a compact variable gain amplifier. Such an amplifier may be applied in the audio field for sound recording or playing devices in telephony, and in particular, at the output of pick-up heads or microphones.
Audio amplifiers must have a high gain and low noise because low signals must be amplified. This type of amplifier is generally made from at least two operational amplifiers connected in series via an attenuator for performing a compression function. The compression function allows the output signal amplitude to be reduced when the input signal amplitude increases too much.
FIG. 1 schematically shows such an amplifier. It represents two operational amplifiers A1, A2 connected in series as inverters through an attenuator A with a variable resistor R40. The operational amplifiers are parallel connected between two supply terminals. One terminal 1 is set to a high potential Vcc and the other terminal 2 is set to a low potential Vee, which is generally ground.
The inverting input of the first operational amplifier A1 receives the input signal Vi to be amplified through a resistor R10. The non-inverting input of the first operational amplifier A1 is set to a DC voltage, which is a fraction of the difference between potentials Vcc and Vee. A voltage divider, which in the example is composed of two resistors R, is connected between the two supply terminals 1, 2. In the example, it is assumed that Vee is ground and that the non-inverting input of the first amplifier A1 is set to Vcc/2 since the voltage divider resistors are equal.
The output of the first operational amplifier A1 is connected to its inverting input through a feedback resistor R20. The output of the first operational amplifier A1 is also connected to the input of attenuator A. The output of attenuator A drives the second operational amplifier A2 via the inverting input thereof. The non-inverting input of the second operational amplifier A2 is set to voltage Vcc/2 just like the first operational amplifier A1. Other voltages can be chosen by giving the voltage divider resistors other values.
The output of the second operational amplifier A2 is connected to the inverting input thereof through a variable feedback resistor R30. The amplifier gain is proportional to (R30xc3x97R10)/(R40xc3x97R20). The choice of the value of resistor R30 makes it possible to adjust the maximum amplifier gain. The output of the second operational amplifier A2 delivers an output signal Vo that is substantially a replica of input signal Vi after amplification.
The signal Vo is a variable component overlaid on a continuous component. The voltage applied to the non-inverting input of the second amplifier A2, which is Vcc/2 in the example, imposes the continuous component. The signal Vo is fed back at attenuator A. When the amplitude of Vi is too high, the value of the attenuator resistor R40 is increased to reduce the amplifier gain and thus the amplitude of Vo.
The attenuator is made from at least one diode (not shown) and, because of the diode, the attenuator does not operate in a perfectly linear way. The delivered signal Vo does not reflect input signal Vi very accurately for certain amplitudes. Also, the level of Vcc/2 cannot be kept perfectly constant when the operational amplifiers are operating.
A mute function may be obtained, but means or a circuit must be provided for disconnecting the input signal from the inverting input of the first operation amplifier. This is not easily implemented. Another disadvantage of the amplifier is size and consequently cost. Portable devices for recording and playing back sound, such as mobile phones, are becoming more miniaturized, their costs are ever decreasing and such amplifiers may be less interesting in such devices in the near future.
In view of the forgoing background, an object of the present invention is to solve such problems of small size, costs, lack of linearity and instability. A variable gain amplifier is proposed that is particularly compact and therefore usable in portable electric devices.
This and other objects, advantages and features according to the present invention are provided by an amplifier comprising an input stage with one or several input terminals for receiving a signal to be amplified, and an output terminal. An inverting gain stage with an input terminal is connected to the output terminal of the input stage. The amplifier further comprises an output terminal for delivering an amplified signal, and a variable feedback resistor connected between the output terminal and the input terminal thereof.
According to the invention, the input stage may be a transconductor stage biased by a current source. A resistor of the current source sets the transconductance in the transconductor stage. Thus, the amplifier may have a gain proportional to the product of the feedback resistor multiplied by the transconductance.
The amplifier is two to three times less bulky than the one previously described. The two amplifier stages are parallel connected between two supply terminals. For the output signal to have a variable continuous component, the gain stage may have another input terminal for receiving a variable DC voltage stabilized with respect to the supply terminal voltages. The input stage may have a long-tail pair of transistors. The current source will be made with transistors so as to deliver a current proportional to VT/R1, where VT is a thermal voltage of kT/q, k is the Boltzmann constant, T is temperature, q is the electron charge and R1 is the resistor determining transconductance.
The gain stage may include a non-inverting long-tail pair of transistors with a non-inverting input terminal forming the input terminal of the gain stage, which is connected to the output terminal of the input stage, and an output terminal. The gain stage may further include an inverting stage with a pair of push-pull transistors and means or a circuit for class-AB biasing the transistors. An input terminal of the inverting stage may be connected to the output terminal of the long-tail pair of transistors of the gain stage. An output terminal forming the output terminal delivers the amplified signal. The feedback resistor is connected between the output terminal of the push-pull transistor inverter stage and the non-inverting input terminal.
According to an alternate embodiment, the gain stage may include an inverting long-tail pair of transistors with an inverting input terminal forming the input terminal of the gain stage, which is connected to the output terminal of the input stage, and an output terminal. The gain stage may further include a tracking stage with a tracking transistor biased by a current source, which is connected to the output terminal of the inverting long-tail pair of transistors, and an output terminal forming the output terminal for delivering the amplified signal. The feedback resistor is connected between the output terminal of the tracking stage and the inverting input terminal. The long-tail pair of transistors of the gain stage may include another input terminal forming the input terminal of the gain stage receiving the variable DC voltage.
Another object of the invention is to provide the amplifier with a compression function and/or a mute function. The biasing source of the input stage may be equipped with means or a circuit for totally or partially bypassing the current delivered thereby. The bypass means may have a long-tail pair of transistors biased by a current source. This long-tail pair has a non-inverting input terminal for receiving a bypass control voltage, an inverting input terminal for receiving a threshold voltage, and an output terminal connected to the current source of the input stage.